1. Field of the Invention
The present invention relates generally to a variable venturi carburetor for an engine in which the cross-sectional area of a venturi portion automatically changes according to the amount of intake air to keep the vacuum generated at the venturi portion at a constant level, regardless of the amount of intake air, the carburetor of this type being called a constant vacuum carburetor. Further, in a carburetor of this type, the metering jet portion of fuel also automatically changes according to the amount of intake air to supply a mixture of a predetermined air-to-fuel ratio. The present invention relates specifically to a variable venturi carburetor of constant vacuum type in which the air-to-fuel ratio can be controlled according to engine operating conditions.
2. Description of the Prior Art
Variable venturi carburetors or constant vacuum carburetors are well known. The variable venturi carburetor is usually attached to an intake passage on the upstream side from a throttle valve. The venturi portion thereof is formed between a fixed venturi portion and a movable venturi portion. The fixed venturi portion includes a nozzle body having a nozzle portion at one end thereof, the nozzle body being connected to a float chamber to supply fuel from the float chamber to the intake passage. The movable venturi portion includes a suction cylinder, a suction piston the inner space of which is partitioned into an atmospheric pressure chamber and a vacuum chamber, and a suction spring.
The suction piston serving as the movable venturi portion moves toward or away from the fixed venturi portion, in dependence upon the force balance determined by pressure difference between the atmospheric pressure chamber and vacuum chamber, the urging force of the suction spring, and the weight of the suction piston, so that the cross-sectional area of the venturi portion changes according to the amount of intake air to keep vacuum at a constant level at the venturi portion. Further, at the center of the lower end surface of the suction piston, a tapered jet needle is fixed so as to pass through a central hole formed in the nozzle body. Therefore, when the suction piston moves toward or away from the fixed venturi portion, the metering jet portion formed between the jet needle and the nozzle portion of the nozzle body varies to keep the mixture obtained at the venturi portion at a predetermined air-to-fuel ratio.
In the prior-art variable venturi carburetor thus constructed, since the stroke of the suction piston is determined according to the amount of intake air and therefore the area of the metering jet portion between the tapered jet needle and the nozzle body is also determined according to the stroke of the suction piston, the air-to-fuel ratio is roughly kept at a constant level, even when the amount of intake air changes. Therefore, there exists a problem in that is impossible to supply a mixture of an appropriate air-to-fuel ratio into the engine according to various engine operating conditions. In more detail, when an engine is running at a low speed and under a heavy load, a rich mixture is preferable for increasing engine power; on the other hand, when an engine is running at a high or medium speed and under a light load, a lean mixture is preferable for saving fuel. However, in the prior-art variable venturi carburetor, since the air-to-fuel ratio is adjusted at a constant level at all times, it is impossible to vary the air-to-fuel ratio freely according to various engine operating conditions.
A more detailed description of the prior-art variable venturi carburetor will be made with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.